Electrosurgical procedures typically rely on the application of very high frequency or radio frequency (RF) electrical power to cut, ablate or coagulate tissue structures. For example, electrosurgery cutting entails heating tissue cells so rapidly that they explode into steam leaving a cavity in the cell matrix. When the electrode is moved and fresh tissue is contacted, new cells explode and the incisions is made. Such electrosurgical cutting involves the sparking of the current to the tissue, also known as the jumping of the RF current across an air gap to the tissue.
Radiofrequency electrodes employed in electrosurgical procedures are generally divided into two categories: monopolar devices and bipolar devices. In monopolar electrosurgical devices, the RF current generally flows from an exposed active electrode through the patient's body, to a passive or return current electrode that is externally attached to a suitable location on the patient's skin. In bipolar electrosurgical device, both the active and the return current electrodes are exposed and are typically in close proximity. The RF current flows from the active electrode to the return electrode through the tissue. Thus, in contrast with the monopolar electrosurgical devices, the return current path for a bipolar device does not pass through the patient's body except for close proximity to the tip of the electrode. Note that bipolar electrosurgical devices are not commonly used in open surgery.
Electrosurgery which takes place in a conductive fluid environment, such as inside of a joint or body cavity filled with, for instance, normalized saline solution, differs from that described previously in that current is conducted from the active electrode through the fluid to the return electrode. In the case of a monopolar device, the current flows through the patient to the return electrode in the manner previously described. In the case of bipolar devices operating in a conductive fluid environment, the return electrode is not in contact with tissue, but rather is submerged in the conductive fluid in the proximity of the active electrode. Current flow is from the active electrode through the conductive liquid and surrounding tissues to the return electrode of the bipolar device. Whether an electrode is monopolar or bipolar, current flows from all uninsulated surfaces of the active electrode to the return electrode anytime that the electrode is energized. This is in contrast to conventional surgery (also called “open surgery”) in which current flows only through electrode surfaces in contact with the patient's tissue.
For an electrode in a fluid environment to vaporize tissue, as in the cutting process described previously, the current density at the electrode/tissue interface must be sufficiently high to insulate the electrode through creation of steam bubbles. Voltage of the electrode must be sufficiently high to cause arcing between the electrode and the patient through the steam bubbles. If such current density and voltage are not achieved, power flows from the active electrode to the return electrode with no desirable clinical effect. In fact, such current flow is highly undesirable since the current flowing from the active electrode heats the conductive fluid in the region surrounding the active electrode. A surgeon using a device which is energized but not arcing to the tissue may believe that he is not affecting tissue in close proximity to the active electrode, however, he may be subjecting the tissue to temperatures approaching 100° C. Even when the electrode is arcing to the tissue, the thermal effects are not limited to vaporization of the tissue. Appreciable undesirable heating of the fluid and tissue in the vicinity to the electrode takes place.
One way of avoiding the negative effects of the undesirable heating of the fluid and adjacent tissue structures is to set the power of the electrosurgical generator to a level that is low enough to minimize the heating of the liquid but high enough to produce sparks. There is an inherent difficulty, however, in satisfying acceptable electrosurgical parameters, since virtually all electrosurgical electrodes are “ignited,” i.e., generate sparks, only when brought into contact with tissue, and then, generally, after a time delay of varying lengths. At the instant when sparks are not generated, most of the RF power supplied to an electrode operating in a conducting fluid is dissipated in the fluid itself as heat, consequently raising the temperature of the fluid within the joint and the adjacent tissue. At the instant when sparks are generated, the RF power is used for the creation of sparks in the vicinity of the electrodes. Therefore, energizing the electrosurgical electrode without initiation of sparks is dangerous and undesirable, as the heating may damage tissue structure uncontrollably in surrounding areas and also deep under the surface.
During the past several years, specialized arthroscopic electrosurgical electrodes also called ablators have been developed for arthroscopic surgery. The ablator electrodes differ from the conventional arthroscopic electrosurgical electrodes in that they are designed for the bulk removal of tissue by vaporization, rather than by cutting the tissue or coagulating the bleeding vessels. This way, during ablation, volumes of tissue are vaporized rather then discretely cut out and removed from the surgical site.
The power requirements of ablator electrodes are generally higher than those of other arthroscopic electrodes. The efficiency of the electrode design and the characteristics of the radio frequency (RF) power supplied to the electrode also affect the amount of power required for ablation. For example, electrodes with inefficient designs and/or powered by RF energy with poorly suited characteristics will require higher powers levels than those with efficient designs and appropriate generators. Probes used in electrosurgery have relatively large area of metallic electrode, which is the active area of the probe. Large electrode area decreases the probe impedance and, therefore, increases the RF power required for proper operation. The shape of the dielectric insulator and of the electrode tip can significantly affect ablation by bubble and spark formation. By properly shaping the insulator and the electrode tip, the threshold power can be substantially decreased.
The amount of fluid temperature increase within a joint and, consequently, the temperature of the adjacent tissue is critical during the use of ablator electrodes. The fluid temperature may easily reach 45° C., at which cell death typically occurs, and this temperature is easily reached with high-powered ablators operating when sufficient flow is not used. The increase in the fluid temperature is also directly proportional to the increase in the power level. As such, the fluid temperature increases as the period of time necessary for an electrosurgical ablator to be energized increases. Standard arthroscopic electrosurgical electrodes are generally energized for only brief periods (generally measured in seconds) while specific tissue is resected or modified. In contrast, ablator electrodes are energized for longer periods of time (often measured in minutes) while volumes of tissue are vaporized.
During ablation, current flow from the ablator into the conductive fluid heats the fluid to its boiling point. Initially, steam bubbles form only at the edges of the ablator, but eventually they cover the entire surface of the electrode. The electrical resistance to current flow increases to its maximum value, maximum voltage is applied to the steam gap and, if the voltage is larger than some critical value, sparking occurs within the bubble. Sparking within the bubble destroys the tissue which is within the same bubble. After the tissue is destroyed, the sparking continues but no beneficial destruction takes place until new tissue is brought into contact with the active region of the probe.
During the time when sparking does not occur, current flow into the fluid during this time causes heating of the fluid with no desirable clinical effect to the patient. Most intensive heating takes place in the region very close to the electrode, where current density is highest. The induced current also heats the liquid and tissue which is a little further away from the immediate vicinity of the electrode tip. This heating is highly undesirable and potentially dangerous as it may damage tissue structure uncontrollably in surrounding areas and also deep under the surface. If higher efficiency probes could be designed, it would lead to less heating of the fluid and it will confer the surgeon a larger safety margin during a specific surgical procedure.
Accordingly, there is a need to minimize the heating of the conductive fluid and especially of the adjacent tissue during an electrosurgical procedure, to achieve improved tissue removal rates at low power levels and to avoid patient burns. There is also a need for an electrosurgical electrode of high efficiency and high impedance with an improved design of the metal electrode tip and of the adjacent dielectric insulator, which is capable of conferring high ablation rates at low RF power levels. A method of fabricating such electrosurgical electrode with advanced electrode tip design and methods of utilizing such electrode in various electrosurgical procedures, are also needed.